This invention relates to an imaging medium and to a process for producing an image. This invention also relates to a polymer useful in the imaging medium of the invention.
So-called "dye diffusion thermal transfer" imaging processes are known which use a receiving sheet and a donor sheet containing a dye. The two sheets are brought close together and the donor sheet is imagewise heated to cause the dye to diffuse to the receiving sheet, on which an image is formed. The donor sheet is then discarded. Full color images can be formed by multiple imagewise heating steps, using a different donor sheet on each pass; for example, a full color image can be formed using cyan, magenta, yellow and (optionally) black donor sheets.
Dye diffusion thermal transfer processes can produce high quality, continuous-tone images, but do suffer from a number of disadvantages. The heating of the donor sheet is typically effected using a thermal head which is scanned in a raster pattern across the donor sheet. The large number of pixels which must be heated on each donor sheet, coupled with the need for multiple donor sheets and for a reasonable total imaging time, require that the residence time of the thermal head on each pixel be short, typically of the order of milliseconds. If a high molecular weight dye is employed, large amounts of energy are required to transfer the dye from the donor sheet to the receiving sheet within this limited residence time. If, on the other hand, a low molecular weight dye is employed, the amount of energy needed for imaging is reduced, but the dye tends to diffuse through the receiving sheet, and such diffusion blurs the image. Low molecular weight dyes also tend to "bleed" from the receiving sheet on to any objects which the image contacts, for example the fingertips of users handling the image, or the pages of albums in which the images are stored.
U.S. Pat. No. 4,957,847 describes a thermal recording medium and process which is effectively a modification of a dye diffusion thermal transfer process. In the process of this patent, color is formed by the reaction between two separate components, namely a benzotriazine compound and a coupling component, which is preferably a naphthol. When used in a dye diffusion thermal transfer type process, one of the benzotriazine compound and the naphthol is coated on the donor sheet and the other is coated on the receiving sheet. Imagewise heating of the donor sheet causes the component thereon to diffuse to the receiving sheet, react with the other component, and form the image. The component which is required to diffuse is of course of lower molecular weight than the dye which forms the final image, and hence the energy requirements for imaging are lowered. However, the colored coupling product is a relatively low molecular weight material which is still susceptible to diffusion within the receiving sheet or on to objects which contact this sheet.
This invention relates to a modification of the imaging medium and process described in the aforementioned patent. The present process allows the diffusible component to be of low molecular weight, while providing a final colored material which is of high molecular weight and is thus much less susceptible to diffusion within or out of the receiving sheet.